the necessary oxygen to the cells. Hemoglobin has a binding affinity for carbon monoxide that is 250 times greater than for oxygen. This is the basis of carbon monoxide poisoning, as hemoglobin can no longer transport oxygen to cells. Cytochromes Cytochromes are heme-containing enzymes that act as single-electron transporters, most notably as electron shuttles in oxidative phosphorylation and photosynthesis. Types of well-studied cytochromes include cytochromes a-c, cytochrome oxidase, and cytochrome P450. These proteins act as electron shuttles by switching the oxidation state of the heme iron atom between ferrous (Fe2+) and ferric (Fe3+). Various cytochromes in combination with other redox-active molecules form a gradient of standard reduction potentials that increases the efficiency of energy coupling during electron-transfer events. Iron-sulfur proteins Iron-sulfur proteins are those with an iron structure that includes sulfur. There are a variety of forms iron and sulfur can take in proteins, but the most common are [2Fe 2S] and [4Fe 4S]. Clusters are often associated with cysteine residues in the protein chain. Non-heme proteins Transferrin Transferrin is found in human plasma, and it is used to traffic and import non-heme iron. It travels freely in the extracellular space. When its iron is needed by the cell, it is brought into the cytosol by a transferrin receptor. Transferrin can bind two Fe(III) ions, along with an anion (usually carbonate). To release the iron, the carbonate anion is protonated. This changes the carbonate's interaction with the protein, changing the conformation and allowing Fe(III) to be transferred. Transferrin has a molecular weight of about 80 kDa. It is a glycoprotein, meaning that it has sugars attached to its amino acid chain. Lactoferrin Lactoferrin is a member of the transferrin family and is the predominant protein found in mammal exocrine secretions, such as tears, milk, and saliva. It is composed of approximately 700 residues and exists mainly as a tetramer, with the monomer:tetramer ratio being 1:4 at 10 μM protein concentrations. The tertiary structure is composed of two lobes, termed N and C lobes, each containing one iron-binding pocket. Each pocket contributes four amino acids (two tyrosines, one histidine, and one aspartate) and, along with two carbonate or bicarbonate anions, forms a six-membered coordinate around the iron cation. It is this specific combination that makes lactoferrin's iron affinity 300 times greater than transferrin. Lactoferrin has significant antimicrobial properties. It is found in the highest concentration of 150 ng/mL in human colostrum